The invention relates generally to superconducting magnet systems and more particularly to local gradient shielding loops for a superconducting magnet.
In one example, an MR system includes a cold mass that comprises a superconducting magnet, a magnet coil support structure, and a helium vessel. Liquid helium contained in the helium vessel provides cooling for the superconducting magnet and maintains the superconducting magnet at a low temperature for superconducting operations, as will be understood by those skilled in the art. The liquid helium maintains the superconducting magnet approximately and/or substantially at the liquid helium temperature of 4.2 Kelvin (K). For thermal isolation, the helium vessel that contains the liquid helium in one example comprises a pressure vessel inside a vacuum vessel.
An MR superconducting magnet typically includes several coils, a set of primary coils that produce a uniform B0 field at the imaging volume, and a set of bucking coils that limit the fringe field of the magnet. These coils are wound with superconductors such as NbTi or Nb3Sn conductors. The magnet is cooled down to liquid helium temperature (4.2 K) so that the conductors are operated at their superconducting state. The heat loads of the magnet, such as that produced by the radiation and conduction from the environment, are removed by either the boil-off of liquid helium in an “open system” or by a 4 K cryocooler in a “closed system”. The magnet is typically placed in a cryostat to minimize its heat loads since the replacement of liquid helium is expensive and since the cooling power of a cryocooler is limited. If the coils are exposed to an AC field, such as an AC field generated by gradient coils of the MR system, AC losses are generated in the superconductors. That is, when superconducting coils are exposed to an AC field, hysteresis loss and eddy currents are induced therein that contribute to AC losses, which can raise the conductor temperatures and potentially cause a quench. The AC losses also add to the total heat load for the refrigeration system. A rise in heat load requires additional cryogenic refrigeration power, which increases operating costs.
Penetration of gradient AC fields into the superconducting magnet should be minimized so that the total heat load of the superconducting magnet can be removed by the refrigeration system. At the same time, the field shielding effects should be very small in the image volume, otherwise the performance of the gradient system is significantly compromised. Use of large volume shield gradients to reduce gradient AC field penetration into the superconducting magnet has a large negative impact on the gradient system performance.
It would therefore be desirable to have an apparatus configured to reduce AC losses caused by gradient AC field penetration into the superconducting magnet with minimal impact on the gradient performance in the imaging volume.